Television scanning system



H. MLLEWIS TELEVISION SCANNING SYSTEM.

April 30, 1940.

INVENTOR VIDEO- FRE ENG! PLIFIER O- SEPARATOR Filed Aug. 10, 1938 DETECTOR NOY TOR

LD M. LEWIS "ATTORNEY Patented Apr. 30, 1940 UNITED STATES PATENT OFFICE I TELEVISION SCANNING SYSTEM Harold M. Lewis, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application August 10, 1938, sen-mm. 224,022 11 Claims. (cine-69.5)

This invention relates to television systems and particularly to scanning-wave generators embodied in such systems and synchronizing means nize the scanning action with other related actions in the system. For example, synchro nizing pulses are usually transmitted as modulation components impressed upon the same television carrier wave with the video-frequency components and these pulses are detected and utilized at the receiver to synchronize its scanning action with the corresponding scanning at the transmitter.

It is important that the synchronization be accomplished with precision in order that proper Scanning may be obtained. This is especially true in scanning systems of the interlaced type, wher the field-scanning frequency is not an integral submultiple of 'the line-scanning frequency so that the lines of one field interlace or fall between the lines of a preceding field and a plurality of successive fields thus constitute a single frame or a single complete image. To obtain a precisely interlaced relation between the successive fields, it is especially important that the field trace scansions be initiated at precisely related intervals.

Heretofore it has been the practice to syrichronize scanning-wave generators at the terminations of each trace-scanning period or, in other words. at the initiations bf each retrace period. This has been accomplished by applying synchronizing pulses to the scanning generators so as to trigger their operation at these points. However, it has been found that, with such synchronization, it is still diflicult to initiate the tracescanning actions at precisely the required points in the scanning cycles.

It is an object of the present inventon, therefore, to provide an improved television scanning system embodying scanning-wave generating means and synchronizing means whereby there may be developed scanning waves having their trace and retrace periods initiated at precisely predetermined times.

In accordance with the present Y invention there is provided in atelevision system adapted for the translation of a signal including scanning synchronizing components occurring during pre determined recurring scanning intervals and bearing a predetermined relationship to initiations of trace-scanning periods, a scanning system comprising an oscillator or generator for developing scanning waves having trace and retrace periods. There are also provided means adapted to be excited by the'signal translated by the system and responsive to the synchronizing components for deriving directly from each of the components an individual synchronizing impulse and for utilizing the derived impulses for synchronizing the generator at the initiation of each trace-scanning period. In accordance with another aspect of the present invention, means are provided for developing from the signal translated by the system, separate synchronizing pulses corresponding to the initiations of the trace scanning periods and for applying these pulses to the generator to synchronize its operation at the initiation of each trace-scanning period. The invention further includes the provision of means for deriving impulses or separate pulses which correspond to the initiations of the retrace-scanning periods and for utilizing the same to synchronize the operation of the generator at the initiation of each retrace-scanning period. v

In the specification and the appended claims, the term pulse is employed in accordance with its conventional meaning to describean amplitude variation in a signal which includes a departure from a given value and a subsequent return to such value after a greater or lesser period; The term impulse, on the other hand, is employed to indicate a single amplitude variation in a signal. For example, in a signal of rectangular wave form, each rectangular pulse comprises a leading edge constituting one impulse and a trailing edge constituting a second separate impulse. Either of these impulses may, of course, represent information or be utilized to effect a controlling'action separately, if desired.

For a better understanding of the present invention, together with other and further objects thereof, reference is hadto the following description taken in connection with the accompanying drawing and its scope will be pointed out'in the appended claims. r

"In the accompanying drawing, Fig. 1 is a circuit diagram, partially schematic, of a complete television receiving system including scanningwave generating and synchronizing apparatus embodying the present invention, while Fig. 2 is a group of curves illustrating the forms of the signal waves utilized and developed in accordance with the presentinvention, to aid in the understanding thereof.

Referringnow more particularly to Fig. 1 of the drawing, there is illustrated a television re-' ceiving system which may be of either the tuned radio-frequency or superheterodyne type and including, in cascade, an antenna-ground system ill, I I, a carrier-frequency translator I2, a detec tor l3, a video-frequency amplifier I4, and an image-reproducing device l5, such as -a cathoderay signal-reproducing tube. Where the receiver is of the superheterodyne type, the conventional frequency changer and intermediate-frequency amplifier are included in the carrier-frequency translator l2. A synchronizing-signal separator I6 is also coupled to the output circuit of the detector l3 and its output circuit is, in turn, connected to a line-frequency scanning-wave generating apparatus I1 and a field-frequency scanning-wave generating apparatus I8; The apparatus l1 and I8 have their output circuits connected to the scanning elements .of the device I5 in conventional manner. is constructed in accordance with the present invention, is shown in detail and hereinafter further described. Those parts of the system indicated in block form may be of any conventional construction and operation.

Since the operation of the system of Fig. 1 as thus far described is, in general, well understood in the art, a detailed explanation of its operation is. unnecessary. Briefly, however, vision-modulated carrier waves are intercepted by the antenna l0,' H and selectively amplified in the carrier-frequency translator I2. Where the receiver is of the superheterodyne type, the signals are impressed on a frequency changer in the translator l2 where they are converted into an intermediate-frequency signal and thereupon further selectively amplifled.' The amplified signal from the translator I2 is delivered to the detector l3,in which there are developed modulation-frequency components including the video.- frequency and synchronizing components. The modulation-frequency components are supplied to the video-frequency amplifier l4, whereinthey are amplified and from which they are applied in the usual manner to a brilliancy control element of the reproducingdevice I5. The modulation- .frequency components are also supplied to the separator l6 wherein the line-synchronizing and field-synchronizing components are separated from the video-frequency components and from each other and from which they are applied to the scanning-wave generating apparatus l1 and I8.

The intensity of the beam of the reproducing device is thus modulated or controlled in accordance with the video-frequency voltages impressed on its control electrode in the usual manner. Saw-tooth scanning waves are developed by the apparatus I1 and J8, controlled by the synchroniz'ing components supplied from the separator l6, as hereinafter further described, and are utilized in the conventional manner to deflect the beam, for example, produce electric fields of saw-tooth wave form to deflect the cathode ray of a signal-reproducing tube, in two directions normal to each other so as to trace the usual scanning The apparatus l8 pattern upon, the screen of the device, thereby to reconstruct the transmitted picture.

Referring now more particularly to the portion of the system of Fig. 1 embodying the present invention, the apparatus It comprises a stabilizer and integrator I9, a diiferentiator and a scanhing-wave oscillator or generator 2|, connected in cascade as shown. While these parts of the system, per se, may be of any suitable construction, in the preferred embodiment of the invention, the stabilizer and integrator l9 comprises a vacuum-tube repeater 22, preferably of the pentode type, having'an input circuit comprising a coupling condenser 23, a leak resistor 24 and a bias battery 25 connected to the output circuit of the separator l6 as shown. The output circuit 'of the tube 22 comprises a resistor 26 and a timeconstant filter circuit including series resistors 27 .with'a suitable fixed bias by the battery 25, that the signal input thereto is stabilized; that is, the

peaks of 'the synchronizing pulses in the signal applied from the separator IE to the control grid level. Where the connection between the control grid and the detector is substantially a direct one, so that the unidirectional component of the signal is not lost, the signal may be sufilciently stabilized without the use of the stabilizing means described. However, where a non-conductive coupling intervenes between the detector and the succeeding stage, as in Fig. l, the unidirectional component is suppressed and the signal tends to center itself about a zero axis in which case, therefore, it must be stabilized as explained. Due to the ,time constant of the anode load circuit comprising resistors 26, 21 and condensers 28, which is long compared with-the duration of the line-synchronizing pulses and line-blanking 'intervals, the voltage developed across the condensers 28 is not of the same wave form as the anode current, but the signal is effectively integrated to produce a voltage wave having pulseseluding an inductance element 32. The grid voltof tube 22 are maintained at a substantially fixed age-plate current characteristic of tube 29 is such I and it is so biased that, with normal input voltage,

' it operates beyond both cutoff limits thereby to limit the voltage wave applied thereto so that a current wave of substantially rectangular-pulse wave form, as indicated at i above the tube, flows in its output circuit through the inductance element 32. That is, the current flowing through the inductance element is of the same wave form as that impressed on the input circuit of tube 29 except that the signal is amplified and is limited at both its upper and lower portions. Since the voltage across the inductance element is the differential of the current therethrough, it is of spaced double-pulse wave form as indicated at en in the figure.

The scanning-wave generator 2| comprises a vacuum tube 33, preferably of the pentode type having relatively sharp upper and lower space:

current cut-oi! characteristics with respect to the outer grid.- The input circuit to the first grid of the tube 33 includes a leak resistor 34 and bat-- cludes aload resistor 3'! and a suitable source of positive operating potential, indicated by the battery 38, The third grid is coupled to the second grid by means of a network comprising a condenser 39 connected directly between the two grids and a resistor 40 and source of negativebiasing potential, for example, battery 4|, connected between the third grid and cathode by way of resistor 44. The values of these elements are such that this network has a time constant of a desired predetermined value, related to that of the saw-tooth wave to be generated, and has a phase shift which is not appreciable; The second grid is maintained at a higher unidirectional potential than the third grid by the battery 38. The generator further includes an-impedance network in the anode circuit of the tube 33 which comprises a load-resister 42 and condenser 43 effectively in parallel and having a time constant related to, that is, of the same order of magnitude as, the periodof the wave to be generated. Operating potential is supplied to the anode by way of the resistor 42 from a suitable point on the battery 38. A resistor 44 is preferably included in the cathode circuit of tube 33 to stabilize the operation of the oscillator and make the bias less critical. I i

A bufier amplifier tube 45 is preferably provided for the generator and comprises a pentode having its input circuit coupled to the output circuit of tube 33, by way of a coupling condenser 45 and leak resistor 41; and its anode circuit cou pled to field-scanningelements of the device; l5 by way of a coupling condenser 48. Operating potentials may be supplied to the screens and anodes of the tubes 22, 29 and 45. from suitable sources indicated at +30 and +3, respectively, the-anode circuit of tube 45 including a suitable blocking resistor 49.

Referring briefly to the operation of the generator 2i, per se, due to the coupling between the second and third grids, the cathode and these grids form a type of oscillator which is well known in-t-he art, the inner grid acting as an anode. Such an oscillator circuit tends to oscillate at a relatively high normal frequency, determined primarily by the distributed inductance and capacitance of the grid circuits, so that the potentials of the second and third grids change rapidly. Due to the time constant of the circuit associated with the anode, however, this natural oscillation is blocked after a fraction of a cycle by the across the anode circuit 42, 43 and this voltage is impressed upon the input circuit of the tube 45 which serves to develop anamplified current of similar wave form and supply it to the deflecting elements of the device IS. The oscillator is .of the detector l3.

particularly well adapted to be synchronized by positive and'negative pulses at the initiation of the trace and retrace periods, respectively, of the wave developed thereby.

The general operation of the system of the present invention may best be explained with reference to the curves of Fig. 2. Curve A illustrates the wave form of one-half the modulation envelope of the received signal, that is, the modulation signal as developed in the output circuit represent the' video-frequency components and the portions indicated at l and I represent the line-synchronizing and field-synchronizing pulses, respectively. The line pulses are doubled during periods immediately preceding and following the field' pulses for the purpose of providing a signal having similar characteristics during the successive field-retrace periods, as well understood in the art. Due to the action of the separator IS, the synchronizing components are separated from the video-frequency signal components so that the signal in the output circuit of the separator is of a wave form such as is illustrated by curve B. This signal may be utilized directly to synchronize the line-frequency scanning generator or, if desired, it may flrstbe differentiated and then applied to this generator in a wellknown manner. 5

The signal developed in the output circuit of The portions indicated at v the separator I6 is, in accordance with the present invention, impressed also upon the input circut of the stabilizer and integrator l9 which operates as explained above to stabilize the signal as applied to the input electrode of the tube 22 and to integrate the signal in its output circuit, thereby to develop a signal having a wave form such as illustrated by curve C, comprising secondary pulses of duration dependent on the leading and trailing edges of the synchronizing-pulse components of the translated signal and including a pulse for each recurring group of field pulses in the applied signal. The integrated signal is thereupon applied to the input circuit of the limiter and differentiator 20 and, due to the characteristic of the tube 29, this signal is limited at both its upper and lower portions and a current wave of substantially rectangular-pulse wave form, such as illustrated by curve D of Fig. 2, is caused to flow in the anode circuit of the tube. As explainedabove, this wave-is differentiated by the inductance element 32 so that a voltage of double-pulse wave form comprising tertiary pulses, such as indicated by curve E, is developed in the output circuit of the tube 29 and applied to the input circuit of the oscillato 2| to synchronize the operation thereof.

As is clearly shown by the curves ,of Fig. 2, the pulses in the integrated signal, curve C, are initiated at points corresponding approximately to the leading edges of the first field pulse of each group of such pulses in the received signal indicated by the arrow above .curve A. Moreover, these pulses of the integrated wave are of a predetermined duration, as determined by the time constant of the integrating circuit, and the limiting tube is so adjusted that the duration of the rectangular secondary pulses derived therefrom corresponds to the desired duration of the re-' trace-scanning periods. Hence a predetermined point, that is, the leading-edge impulse of. each rectangular secondary pulse, corresponds to the termination of a trace period and to the initiation of a retrace-scanning period and the trailing-edge impulse of each rectangular secondary pulse corresponds to the initiation or a tracescanning period. The differentiated signal or double-pulsewave shown at E, therefore, in-. cludes individual tertiary pulses corresponding to the initiation of each retrace- .and each tracescanning period. This wave is applied to the generator 2| in the polarity indicated in Fig. 2, which is the proper one to provide a negative pulse for the initiation of each retrace period and a separate positive pulse for the initiation of each trace-scanning period. Thus'the operation of the generator 2| is triggered at these points 7 and the initiations of the trace periods as well as the retrace periods are precisely timed. The saw-tooth wave developed by the generator II is illustrated by the curve F of Fig.2. This signal is applied to the output amplifier and repeated thereby as a current and supplied to the scanningelements of the cathode-ray tube l5, as explained above,- to effect the field-scanning action in conventional manner.

It will be seen from the above that the present invention provides a system which is especially adapted for the translation of a signal whichincludes synchronizing components occurring during predetermined recurring scanning intervals and bearing a predetermined relation to the initiations of the trace-scanning periods. These components are represented, for example, by each group of field pulses in the received signal. Moreover, the invention provides means responsive to these components in the signal translated by the system for deriving directly from each of the components an individual synchronizing impulse, such impulse being represented in the signal illustrated by curve D by the trailing edge of the rectangular pulse indicated at y or by the leading edge of the pulse 2 shown in curve E. It is further to be noted that the system of. the present invention comprises means responsive to synchronizing components in the signal input to the system for developing .a separate synchronizing pulse, the pulse 2 of Fig. 2, corresponding to the initiation of each trace-scanning period and for applying such pulse to the scanning-wave generator for synchronizing its operation at the initiation of each trace-scanning period. Likewise, impulses and/or pulses which correspond to the retrace-scanning periods while the trailing-- edge impulses initiate the trace-scanning periods. While there has been described what is at present'considered to be a preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:-

1. In a television system adapted for the translation of a signal including synchronizing components occurring during predetermined recurring scanning intervals and bearing a predetermined relation to initiations of trace-scanning,

- periods, a scanning system comprising an oscillator for developing scanning waves having trace-. and retrace-scanning periods, means adapted to be'excited by said signal translated by the system and responsive to said synchronizing components tor deriving directly from each of said components an individual synchronizing impulse, and'means for utilizing said derived impulses to synchronize said oscillator at the initiation of each trace-scanning period.

2. In a television system adapted for the translation of a signal including synchronizing components'occurring during predetermined recurring scanning intervals and bearing a predeter-v mined relation to initiations of trace-' and retrace-scanning periods, a scanning system comprising an oscillator for developing scanning waves having traceand retrace-scanning periods, means adapted to be excited by said signal translated by the system and responsive. to said synchronizing components for deriving directly from each of said components an individual synchronizing impulse, and means for utilizing said impulses to synchronize said oscillator at the initiation of. each traceand retrace-' scanning period.

3. In a television system adapted for the translation oi. a signal including synchronizing components occurring during predetermined recurring scanning intervals, a scanning system comprising an oscillator for developing scanning waves having traceand retrace-scanning period's, means adapted to be excited by said signal translated by the system and responsive to said synchronizing components for developing separatesynchronizing pulses c'orresponding to the initiations of trace-scanning. periods, and means for applying said pulses to said oscillator to synchronize its operation at the initiation of each trace-scanning period. I

4. In a television system adapted for the translation of a signal including synchronizing components occurring during predetermined recurring scanning intervals, a'scanning system comprising an oscillator for developing scanning waves having traceand retrace-scanning periods, means adapted to be excited by said signal translated by the system and responsive to said synchronizing componentsfor developing separate pulses corresponding to the initiation oi each traceand retrace-scanning period, and

means for applying said pulses to said oscillator -to synchronize its operation'at the initiation of each traceand retrace-scanning period.

5. In a television system adapted for the translation of a signal including synchronizing components occurring during predetermined recurring scanning intervals and bearing a predetermined relation to the initiations of'trace-scanning periods, a scanning system comprising. an oscillator for developing scanning waves having traceand retrace-scanning periods, means adapted to be excited by said signal. translated by the system and responsive to said synchronizing components for deriving directly from each of said components an individual synchronizing pulse, and means for applying said derived pulses to said oscillator to synchronize its I operation at said initiation of each trace-scan.-

ning period.

6. In a television system adapted for the translation of a signal including synchronizing components occurring during predetermined recurring scanning intervals and bearing a predeter mined relation to the initiations of retraceand trace-scanning periods, a scanning system ,com-

prising an oscillator for developing scanning waves having traceand retrace-scanning periods, means adapted to be excited by said signal translated by the system and responsive to said synchronizing components for deriving directly from each of said components an individual synchronizing pulse, and means for applying said derived pulses to said oscillator to synchronize its operation at the initiation of each traceand retrace-scanning period.

'7. In a television system adapted for the translation of a signal including synchronizing pulses occurring during predetermined recurring intervals and bearing a predetermined relation to the initiations of traceand retrace-scanning pe riods, a scanning system comprising an oscillator tor developing scanning waves having traceand retrace-scanning periods, means adapted to be excited by said signal translated by the system and responsive to said pulses for developing separate groupsof synchronizing pulses corresponding to said initiations of said traceand retracescanning periods, means for utilizing one of said groups of pulses for synchronizing said oscillator at the initiation of each trace-scanning period, and means for utilizing the other of said trains of pulses for synchronizing said oscillator at the initiation of each retrace-scanning period.

8. In a television system adapted for the translation of a signal including synchronizing pulses, certain of said pulses having leading edges representative of the initiations of retrace-scanning periods and certain of said pulses having trailing edges bearing a predetermined relation to the initiations of trace-scanning periods, a scanning system comprising an oscillator for developing a scanning wave having traceand retrace-scanning periods,'means responsive to said synchronizing-pulse leading edges in the signal translated by the system for deriving a first group of. individual pulses, means adapted to be excited by said signal translated by the system and responsive to said synchronizing-pulse trailing edges for deriving a second group of individual pulses, means for utilizing said first group of derived pulses for synchronizing said oscillator at the initiation of each retrace-scanning period, and means for utilizing said second group of derived pulses for synchronizing said oscilator at the initiation of each trace-scanning period.

9. In a television system adapted for the translation of a signal including synchronizing-pulse components having leading edges representative said integrated of each retraceperiods and trailing edges of the initiations oi retrace-scanning periods and trailing edges bearing a predetermined relation to the initiationsof trace-scanning periods, a scanning system comprising an oscillator veloping a scanning wave having traceand retrace-scanning periods; means adapted to be excited by said signal translated by the system for integrating said components,.means for limiting components, mea'nsior differentiating saidlimited integrated components to derive oppositely poledpulses corresponding to said leading and trailing edges of the synchronizing-pulse components of said signal, and means for utilizing said oppositely poled pulses for synchronizing said oscillator at the initiation and trace-scanning period.

10. In a television system adapted for the translation of a signal including synchronizingpulse components having leading edges representative of the initiations of retrace-scanning bearing a predetermined relation to the initiations of trace-scanning periods, a scanning system comprising an oscillator for developing a scanning wave having traceand retrace-scanning periods, means adapted to be excited by said signal translated by the system for deriving secondary pulses of predetermined duration dependent on said leading and trailing edges of said pulse components in said translated signal, and means for utilizing the leading and trailing edges of. said derived pulses of predetermined duration for synchronizing said oscillator at the initiation of each retraceand trace-scanning period.

11. In a television system adapted for the translation of a signal including synchronizingpulse components having leading edges representative of the initiations of retrace-scanning periods and trailing edges bearing a predetermined relation to the initiations of trace-scanning periods, a scanning system comprising an oscillator for developing a scanning wave having traceand retrace-scanning periods, means adapted to be excited by said signal translated by the system for deriving secondary pulses of pre- 1 determined duration dependent on said leading and trailingedges of said pulse components in said translated signal, means for deriving, from said secondary pulses, tertiary pulses determined by the leading and trailing edges of said secondary pulses, andmeans for utilizing said tertiary pulses forsynchronizing said oscillator at the initiation of each retraceand trace-scanning period.

HAROLD M. LEWIS.

for dc- 

